8600GTS is only good at benchs, but in practice is under 7900GS, a bit more than a 7800GT and it performance drops drastically when filters are applyed. We don´t know how nvidia 8 series perform at Dx10... and Dx10 era is too far for thinking on it

Bioshock gets the same FPS in DX9 as DX10 on an 8600GTS, and puts in performance between a 7900GS and 7900GT...

Yeah, the image quality is nearly the same with D9 or D10, it is just a port, not a real D10 game.

BioShock isn't a DX10 port, it's the first native DX10 game and so far it's looking REALLY good for the 8600...

Don't forget driver issues, expect DX10 improvement once drivers get more developed, the only thing I can actually say that truely is bad about the 8600 GTS is the 128bit bus, thats gonna kill your fps at high resolution, with alot of AA and AF.
~Bryan~

I am not sure Bioshock is native DX10... the game is DX9 with the option of activate some DX10 effects...
Anyway 7900GS or 7900GT are weak cards for today games, IMO

It's got a native DX10 mode though - you can have a game built "from the ground up" with DX10 AND DX9 support.